Compute and show ETA for vault tasks

When requested object objects are present in swh-graph, we should be able to approximate the total runtime of vault tasks, as I expect it to be linear in the number of objects of each type.

How to do it:

1. use data in swh-scheduler's database to get the run time of cooking each root object
2. use swh-graph to compute the number of objects of each type (cnt + dir + rev should be enough) reachable from that root object
3. run a linear regression to obtain a model of the runtime as a function of the number of object of each type
4. store that model somewhere (vault backend? swh-web?)
5. every time we get a cooking request, query counts in swh-graph (like in step 2) and use the model to estimate the run time

As a first approximation, we could skip steps 3 and 4, this might be good enough, as the git-bare cooker fetches objects somewhat homogeneously (ie. a batch of revs, then a batch of dirs, then a batch of contents, then revs again, ...)

This would be a great UX improvement, as some gitfast/git-bare tasks can be really long.

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Migrated from T3550 (view on Phabricator)